Vocoders are designed to compress voice signals to lower data rates by utilizing certain voice characteristics. In Code Division Multiple Access (CDMA) systems variable rate vocoders are utilized, which vary the data rate based on the current speech activity. One such currently deployed vocoder is the Enhanced Variable Rate Codec (EVRC) specified in IS-127. EVRC utilizes three data rates: full, half and eighth rate. Typically full rate is used when there is speech activity, eighth rate when there is no activity and is usually representative of the background noise. Half rate is used for transition periods and also when signaling/control information needs to be multiplexed with the speech information. Note that EVRC is an example of a Rate Set 1 vocoder, corresponding to a maximum data rate of 8 Kbps. CDMA also supports Rate Set 2 vocoders corresponding to a maximum data rate of 13 Kbps.
It is well known that in CDMA systems the capacity of the air interface is inversely proportional to the power transmitted by the constituent channels. Thus it is beneficial to keep power transmission to a minimum. However, the lower the transmission power the more likely a packet is to be lost, or erased, on the air interface. Excessive packet loss will result in unacceptable voice quality. A power control algorithm is therefore used to dynamically adjust transmission power to maintain a fairly constant rate of erased frames termed a Frame Erasure Rate (FER). Typically a voice channel is maintained at an erasure rate of approximately 1% so as to maintain adequate voice quality.
By introducing retransmission capabilities the transmission power can be decreased while maintaining an overall acceptable voice quality. For example while operating at a FER of 10%, 1 in 10 frames will be erased on original transmission but because the erased frames are retransmitted an overall voice packet erasure rate of 1% (10%*10%) is achieved. However, there is only limited channel bandwidth available and there may not be bandwidth available to retransmit the packet as well as transmit a current packet. For example if the retransmitted packet was Full Rate and the current packet was also at Full Rate.
Thus a method of retransmitting a voice packet in a limited bandwidth channel while maintaining adequate voice quality is desirable.